I think if you want to change it should be something equally exquisite otherwise visually it will be a let-down.
and a cocktail dress ? I think there will be many other occasions to wear them.
Re: 2 dresses??? Should change it during the party???
can’t you bustle your longer dress so that its easier to move/dance in?
and i agree with majesty, if you go with a second, it should be as lovely as the first and that second dress is kind of meh.
have you seen these Amsale dresses? they’re GORGEOUS and so sophisticated! http://www.amsale.com/lwd/lwd1.shtml